This invention relates to a variable camshaft assembly in which one set of cam lobes can be moved relative to a second set of cam lobes.
Camshaft assemblies are known which comprise a tube to which some of the cam lobes are fixed and about which other cam lobes are free to rotate. A drive shaft that passes through the bore of the tube and connecting pins that pass with clearance through an aperture in the tube couple the rotatable cam lobes to the drive shaft. The angle of the individual cam lobes can thus be controlled by setting the angle of the drive shaft relative to the drive tube.
In order to minimise any angular variation between the cam lobes attached to the drive shaft, it is advantageous to retain the connecting pins in the shaft via an interference fit, whilst the connecting pins have a small clearance in the cam lobes. If the clearance fit were to be located at the interface between the pins and the drive shaft, a more significant angular variation would result.
It should be noted that the connecting pins cannot be an interference fit in both the drive shaft and the cam lobe as small tolerance variations would result in the assembly becoming locked. The cam lobe axis of rotation is defined by the sliding fit on the outer surface of the tube and the drive shaft is required only to give angular alignment of the cam lobe. If there were to be no clearance in the system, the drive shaft would also attempt to determine the cam lobe axis of rotation and hence small tolerance variations would prevent the assembly from rotating freely.
In order to allow the assembly of the connecting pins into the drive shaft, it is necessary to ensure that the axial force applied to the pin to overcome the interference fit in the drive shaft cannot cause the shaft to bend beyond its elastic limit. This has been achieved in the past by providing bearing areas on the drive shaft that are a running fit in the bore of the tube adjacent to each of the connecting pins. The assembly can then be supported on the cam lobe or the tube whilst the connecting pins are pressed into position without the drive shaft becoming distorted.
Whilst this design has been demonstrated to be a successful method of producing a camshaft assembly of this type, it does have two disadvantages, namely:
The whole bore of the tube must be accurately positioned to its outer diameter and finished to an accurate diametral tolerance.
A number of bearing areas need to be finished on the shaft to an accurate tolerance and these are subsequently redundant once the components have been assembled since only two bearings are necessary to position the shaft in the bore of the tube.
These two factors can have a significant effect upon the ease of manufacture and hence the cost of the assembly and the aim of the invention is to mitigate these problems.
In accordance with a first aspect of the invention, there is provided a method of assembling a variable camshaft assembly having a first cam lobe that can be moved relative to a second cam lobe, the assembly comprising a tube to which the first cam lobe is fixed and about which the second cam lobe is free to rotate, a drive shaft that passes through the bore of the tube and a connecting pin that passes with clearance through an aperture in the tube to couple the second cam lobe for rotation with the drive shaft, which method comprises inserting into holes in the cam lobe and in the drive shaft a hollow connecting pin having a constant outer diameter dimensioned to be a close fit in the holes, and expanding the outer diameter of the connecting pin only within the region of the pin that lies within the drive shaft so that an interference fit is generated with the drive shaft.
Preferably, the connecting pin has an inner diameter that varies along its length, being larger at its end engaging the cam lobe than at its region in line with the drive shaft, and the outer diameter of the pin is expanded by inserting into the pin an element of larger outer diameter than the smaller inner diameter region of the connecting pin.
The element used to expand the pin may be a pin, a ball or a screw that remains within the pin after assembly is completed. Alternatively, it may be a mandrel that is withdrawn from the pin after it has locally stretched the pin beyond its elastic limit.
In accordance with a second aspect of the invention, there is provided a variable camshaft assembly having a first cam that can be moved relative to a second cam, the assembly comprising a tube fast in rotation with the first cam and rotatably supporting the second cam and a drive shaft disposed within the tube and coupled for rotation with the second cam by means of a connecting pin that passes with clearance through a hole in the tube, wherein the connecting pin is a hollow pin that is a sliding fit in the second cam and that is expanded in situ to form an interference with the drive shaft.
Because the pin can be expanded into an interference fit with the drive shaft without applying an excessive force tending to bend the drive shaft, it is no longer necessary for the drive shaft to be supported along its entire length and it can instead, in accordance with a preferred feature of the invention, be journalled in the surrounding outer tube at only two locations, preferably its axial ends, leaving a clearance between the drive shaft and the tube over the major proportion of its length. Such a clearance obviates the need for the entire bore of the tube and the outer surface of the drive shaft to be accurately machined.
The bearings may each take the form of an inserted bush, or the drive shaft could run directly against a machined surface inside the tube.